1. Field of the Invention
This invention relates to reversible heat-sensitive recording materials in which formation of images and erasion of the images can be carried out by controlling the heat energy.
2. Prior Art
Heat-sensitive recording materials generally comprise a support and, provided thereon, a heat-sensitive recording layer mainly composed of a normally electron donating colorless or slightly colored dye precursor and an electron accepting color developer. The dye precursor and the color developer instantaneously react upon application of heat by thermal head, thermal pen, laser beams or the like to form an image. Such heat-sensitive recording materials are disclosed in Japanese Patent Application Kokoku Nos. 43-4160, 45-14039 and the like.
In general, in the case of these heat-sensitive materials, when an image is once formed, it is impossible to erase the image to restore the portion to the original state.
Therefore, for further recording of information, it is only possible to make recording in the portions where no image is formed. Accordingly, the area for heat-sensitive recording is limited and the information to be recorded is restricted and not all of the necessary information can be recorded.
Recently, reversible heat-sensitive recording materials capable of repeating the formation of images and the erasion of the images have been proposed for solving the above problems. For example, Japanese Patent Application Kokai Nos. 54-119377, 63-39377, 63-41186 and the like, disclose heat-sensitive recording materials comprising a matrix resin and an organic low-molecular compound dispersed in the matrix resin. However, in these recording materials, the transparency of the recording materials is reversibly changed and so the contrast between the imaged portion and the unimaged portion is insufficient.
Furthermore, according to the methods described in Japanese Patent Application Kokai Nos. 50-81157 and 50-105555, since the images formed by these methods change depending on the environmental temperatures, the temperature at which the image-formed state is maintained differs from the temperature at which the image-erased state is maintained and so these two states cannot be maintained for a desired period at room temperature.
Further, Japanese Patent Application Kokai No. 59-120492 mentions a method for maintaining the image-formed state and the image-erased state by keeping the recording material in the region of the hysteresis temperature utilizing the hysteresis characteristics of color forming components. However, this method has the defects that a heating source are needed for formation and erasion of images and besides, the temperature region at which the image-formed state and the image-erased state can be maintained is limited to the region of the hysteresis temperature. Thus, this method is still not sufficient for using the materials in the temperature environment of daily life.
In addition, Japanese Patent Application Kokai Nos. 2-188293 and 2-188294 and International Patent Publication No. WO90/11898 disclose reversible heat-sensitive recording media comprising a leuco dye and a color developing and decolorizing agent which causes color formation of the leuco dye upon heating and causes erasion of the color. The color developing and decolorizing agents are amphoteric compounds having an acidic group which causes color formation of the leuco dye and a basic group which causes decolorization of the leuco dye and they preferentially cause one of the color formation action of the acidic group and the decoloration action of the basic group by controlling the heat energy, thereby to perform the color formation and decolorization. However, according to this method, it is impossible to completely exchange the color forming reaction and the decolorizing reaction from each other only by control of heat energy and since both the reactions simultaneously take place at a certain ratio, sufficient color density cannot be obtained and besides the decolorization cannot completely performed. For this reason, a sufficiently high contrast of the image cannot be obtained. Moreover, since the decolorizing action of the basic group acts also on the color formed portion at room temperature, the density of the color formed portion inevitably decreases with time.
Furthermore, in Japanese Patent Application Kokai No. 5-124360, reversible heat-sensitive media which can form images and erase images by heating a leuco dye is described, and as an electron-receiving compound, there are exemplified an organic phosphoric acid compound, .alpha.-hydroxy-aliphatic carboxylic acid, aliphatic dicarboxylic acid and a specific phenol compound such as an alkylthiophenol, an alkyloxyphenol, an alkylcarbamoylphenol, alkyl gallate each having a carbon number of 12 or more, etc. However, in this recording media, coloring density is low or erasion of images becomes incomplete so that the two problems cannot be solved simultaneously and image stability with time is also not practically satisfied.
As explained above, according to the conventional technique, there have been no reversible heat-sensitive recording materials which can give good image contrast, can form images and erase the images and can maintain images having time stability under the daily environment.
The present inventor has found a novel reversible colorization agent which can give good image contrast, can form images and erase images and can maintain images having time stability under the daily environment and proposed in Japanese Patent Application Kokai No. 6-210954. However, in the reversible heat-sensitive recording materials having a recording layer which forms images by heating, it is recognized that the properties such as color developing, decolorizing and repeated properties thereof, etc. are effected by a binder resin or a pigment other than the color developing components, and depending on the kinds of the resin or pigments, there sometimes causes practical problems. That is, there are problems that in the reversible heat-sensitive recording materials, if forming images and erasing images are repeated many times at the same portion, reversible color developing agent particles are liable to aggregate by heat and pressure when heat is applied and lowering in coloring density and deterioration of a recording layer occur by the repeated use.